Electrodynamic sound-reproducing device



Feb- 3, 1932.' L. w. MURKHAM ET AL 1,846,351

ELECTRODYNAMIC SOUND REPRODUCING DEVICE Filed Dec. 25, 1930 Patented F eb. 23, 1932 UNITED STATES PATENT OFFICE LEONARD WALTER MURKHAM AND DOUGLAS TEMPLE BENNETT, OF LONDON, ENGLAND ELECTRODYNAMIG SOUND-REPRODUCING DEVTCE Application filed December 23, 1930, Serial No. 504,332, and in Great Britain June 13, 1980.

the centre pole and be contained within the magnet, and the flux which can be produced in the gap is limited by the resulting limitation in the size of the field coil. It is wellknown that the flux density within the annular gap decreases as the radial width of the gap is increased, and it is therefore desirable from this point of view to employ a magnethaving a gap of small radial width; such a small gap is especially desirable where t t 1 i 1 1 peimanen magne s are emp oyec, anc pro viding a moving coil can be centered accurately for axial movement within the gap of a permanent magnet the arrangement is found to be as eflectiveas many of those with electromagnets having comparatively large gaps.

In loud speakers of the kind in question, the

Vibratory member or diaphragm is as a rule supported by means of a more or less flexlble surround in such manner that little or no air leakage can take place from the front to the back of said member, in order that the maximum volume of air may be set in motion; and it is apparent that if a vibratory member, which with its surround is of but little greater diameter than the annular gap of the magnet, and which in fact is secured to or mounted upon the magnet itself, be employed, it will be readily possible to maintain a very accurate centering of a moving coil on the vibratory member with regard to the annular gap, whereas with a vibratory diaphragm such as a cone, supported at some distance from the gap, considerable difficulty will be experienced. With the former arrangement,

however, there is the danger of air being trapped either between the diaphragm and the centre pole piece or within the casting of the magnet.

The object of the present invention is to provide an improved arrangement whereby such trapping of air, and the consequent deleterious effects upon the fidelity of sound reproduction, may be avoided, and also to provide an improved vibratory member and moving coil assembly for use with a narrow gap magnet.

In an electro-dynamic or moving coil sound-reproducing device according to the present invention, the centre pole of the magnet, of which latter the gap is of small radial Width, is formed at its outer face with a cavity and with bores or holes extending diagonally from this cavity tothe curved surface of said pole.

A vibratory member for use with such a magnet preferably extends over or covers the annular ap and outer face of the centre pole, whilst the cavity at the outer face of said pole contains light yielding material, such as cotton wool, in sufficient quantity to project therefrom yieldingly to bear against the rear surface of the vibratory member. This cotton wool or other material serves to damp the movement of the vibratory member and to prevent resonances or reverberation.

For centering the vibratory member and moving coil assembly accurately with regard to the annular gap of the magnet, said assembly is preferably mounted upon an annulus or apertured plate, which latter in turn is secured to the magnet by means of screws passing through oversize holes formed in the annulus or plate, or, alternatively, in the magnet, to allow the annulus or plate, and with it the vibratory member and moving coil assembly, to be shifted laterally relatively to the magnet and the annular gap of the latter.

Preferably, that part of the vibratory member which, when in position, is immediately opposite to the outerface of the centre pole of the magnet, is of domed formation, the dome extending away from the outer face of said centre pole. The vibratory member may comprise, for example, a centre piece formed from light rigid material, such as duralumin, as a dome-shaped pressing, the maximum diameter of which is substantially equal to the mean diameter of the annular gap of the magnet, with a small flange which is secured to a surround of flexible, or semi-flextor-y member-centre piece or pressing at the junction of the flange with the domed portion thereof. 1

It will be observed that an arrangement according to the'presentinvention is particularly applicable to those sound-reproducing devices in which a small vibratory diaphragm serves to-energize an air column by .means of'a-horn, and a magnet according to one formof thepresent invention for such a the coil '27 and former. 28 within the annular gap 16,- screws 31 extend through oversize bores 32 formed :in the plate 17 into the loud speaker'is illustrated by'the accompanying drawings, of which Figure 1 is a sectional elevation Figure-2 is'a corresponding inverted plan view in which the magnet is purposely omittedwhilst Figure 3is a plan view showing the magnet poles.

As shown, the annular poleofthe magnet is indicated at 11 and the-cylindrical centre pole at 12, the latter being formed at its end facewith .a cavity 13, from which fourbores :orholes ltextend'to'the curved surface-of the pole 12, which in the particular case illustrated, is of reduced, diameter, as indicated at 15 (Figure 1 nearits end.

In the drawings the radial width'of the annular gap indicated at 16, and the thickness-of the vibratory and'associ'a'ted parts is exaggerated purposely for'the sake of clearness. All the vibratory parts are mounted upon a plate 17'for med with a hollow raised portion 18,'from which aneck 19 eXtends centrally for theattachment of a horn (not shown) ,the diaphragm proper beingiformed, as a duralumin pressing 20,as a dome with a small radial"flange"21.

Asurround'22 is in'thel form of a'ring cut from thin sheet celluloid, secured'by-adhesive at its inner margin to the flange 2-1 of the diaphragm 20, and clamped at its outer man gin between two paper or pasteboard washers 23 by means of ametal clamp-ingrin 24and countersunk headed screws 25 (Figure 2) which pass'through the clamping member '24 into the plate 17 It will be observed that in view of the thickness of the clamping ring 24, thefiange .Zland the surround 22 will be slightly nearer theadj'acent face of-the'pla-te 17 than the face of theannular pole 11 of the magnet. To provide the necessary'c'learance, the face of the plate 17 is chamfered, asshown at 26,in

the neighbourhood of the hollow raised por- "tion 18 thereof. The moving coil, indicated at27,-cons1sts'of twolayers-of fine aluminium axially from the diaphra m-2O to the junctionofthedome and the ange 21 thereof.

For damping the motion of the diaphragm and for avoiding resonance or reverberation from'the outer 'face'13'of'thecentrepo1e"12, a

wad -ofcotton wool,-or other light yielding substance, is secure'd by adhesive to the outer .face 18 of the centrepole 12,.this wad being sufficiently large to be compressed slightly between said pole 112 and .the inner face of the I diaphragm .20 when thealatter is in :position.- I

To provide for the :accurate centering of threaded bores '33 formedzfor itheipurposedn the outer pole o'f:.the magnet,theheads of;said screws being retained within oversize countersunk bores 34 in saidtplate 17.

In practice,-the moving coiland diaphragm to the magnet until the coil.27'iscentred-accurately in the gap16. Once the proper -location-has been found,-'the screws 31:aretightened up and the device is ready for use. In view of the'bores 14 extending from thecav- :ity 13iof the: centre vpole .-12,;the gap between the pole face :13 anddiaphrag1n20=cannotbecome compressed bythezmotion ofithe latter,

despite the very small clearance between the paper former 28 and the inner wall of the :annular gap .16.

'Itisto be observed thatthe successful :use

.of asmall.gap magnetwhich is rendered possible by the 'presentinvention, enables per manent magnets tobe employed -assuccessfully :as electro-magnets,-so that-the troubles due to the development of heat in the winding of:an electro+magnet owing to :the gPfiS- sage of a :heavv current therethrough, may

also be-aVoidedI-Whilstitherecan be produced a loud speaker verycmuch more robust than those in'common use'in which there is, practically speaking, no risk of the movingacoil rubbing againsteither of the :pole piecesand thus becoming damaged, despite the extremely minute ;gap which can 'be employed,

and finallylthat with all these advantagesia sound-reproducing device according to the present invention is capable of reproducing .sound withequal, if'not-greater, fidelity than those of the more usualwtype.

What we claim is 1.- In an electro dynam'ic or moving coil sound-reproducing device, a :magnet ha i 'a cylindrical annular gap of small radial width, a moving coil mounted for axial motion in said gap, the magnet including a center pole having its outer face formed with a cavity, said center pole being formed with a plurality of bores leading from said center cavity to the outer surface of the center pole.

2. In an electro-dynamic or moving coil sound-reproducing device, a magnet having a cylindrical annular gap of small radial width, a moving coil mounted for axial motion in said gap, the magnet including a center pole having its outer face formed with a cavity, said center pole being formed with a plurality of bores leading from said center cavity to the outer surface of the center pole, said bores being substantially radial of the center pole and at an angle to the axis of the center pole.

3. In an electro-dynamic or moving coil sound-reproducing device, a magnet having a cylindrical annular gap of small radial Width, a moving coil mounted for axial motion in said gap, the magnet including a center pole having its outer face formed with a cavity, said center pole being formed with a plurality of bores leading from said center cavity to the outer surface of the center pole, and a vibratory member overlying the annular gap and outer face of the center pole, said vibratory member being of substantially dome form.

4. In an electro-dynamic or moving coil sound-reproducing device, a magnet having a cylindrical annular gap of small radial width, a moving coil mounted for axial motion in said gap, the magnet including a center pole having its outer face formed with a cavity, said center pole being formed with a plurality of bores leading from said center cavity to the outer surface of the center pole, a vibratory member overlying the annular gap and outer face of the center pole, said vibratory member being of substantially dome form, and yielding material arranged in the cavity at the outer face of the center pole and bearing yieldingly against the vibratory member.

5. In an electro-dynamic or moving coil sound-reproducing device, a magnet having a cylindrical annular gap of small radial width, a moving coil mounted for axial motion in said gap, the magnet including a center pole having its outer face formed with a cavity, said center pole being formed with a plurality of bores leading from said center cavity to the outer surface of the center pole, a vibratory member overlying the annular gap and outer face of the center pole, said vibratory member being of substantially dome form, an element carrying the vibratory member and moving coil, and means for adustably mounting said element with respect to the magnet to provide for centering the vibratory member and moving coil with respect to said cylindrical annular gap.

6. In an electro-dynamic or moving coll sound-reproducing device, a magnet having a cylindrical annular ap of small radial width, said magnet inc uding a center pole and a vibratory member comprising a central section of dome shape having a maximum diameter substantially equal to the mean diameter of the annular ga magnet, said vibratory member inc uding a flange, a non-resonant section secured to and projecting beyond the flange, and an element to which said section is secured.

7. In an electro-dynamic or coving coil sound-reproducing device, a magnet having a cylindrical annular gap of small radial width, said magnet including a center pole and a vibratory member comprising a central section of dome shape having a maximum diameter substantially equal to the mean diameter of the annular gap of the magnet, said vibratory member including a flange, a nonresonant section secured to and projecting beyond the flange, an element to which said section is secured, and a moving coil including a cylindrical former and a winding thereon, said moving coil being secured to the vibratory member at the juncture of the dome and flange thereof.

LEONARD WALTER MURKHAM. DOUGLAS TEMPLE BENNETT.

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